The present invention relates to a method for the synthesis of 2-(4-hydroxyphenoxy)alkanoic acids. Such compounds are useful in the production of herbicides and dyes. It is known in the art to produce herbicidal agents which are of the 2-(aryloxyphenoxy)alkanoic acid class. Within this context, aryl includes phenyl, pyridyl, benzoxazolyl, etc.
These and other compounds are more fully described in U.S. Pat. Nos 4,589,908; 4,130,413; 4,391,995; 4,301,295; 4,238,626; 3,784,697; 3,721,703; 3,954,442; 4,657,577; 4,629,493; 4,046,553; and 4,368,068, all of which are incorporated herein by reference.
The production of these herbicides requires the use of an intermediate which is a 2-(4-hydroxyphenoxy)alkanoic acid or ester (I) of the formula: ##STR1## wherein the variables are hereinafter defined. However, prior processes for producing these intermediate compounds have employed hydroquinone and other compounds as starting materials. Mono-o-alkylation of hydroquinone is achieved by using a large excess of hydroquinone, but this method warrants low conversion. Alternatively, one can make mono-o-protected hydroquinone, alkylate, and remove the protecting group. However, the cost of such a manufacturing procedure is very large. Mono-o-alkylated hydroquinone derivatives, such as 2-(4-hydroxyphenoxy)propionic acid are difficult to obtain because both of the hydroxyl groups of hydroquinone tend to react with the alkylating agent. Such processes are discussed at length in U.S. Pat. Nos. 3,600,437; 4,532,346; 4,547,583; 4,613,677; 4,489,207; 4,368,068 and British Pat. No. 1,591,063. U.S. Pat. No. 4,665,212 teaches the condensation of hydroquinone or hydroquinone salts with certain aromatic sulfonyl containing acids, esters and salts. U.S. Pat. No. 4,511,731 teaches the preparation of propanoate monoethers of hydroquinone via sequential oxidation of hydroxystyrene. While such processes are effective for producing herbicide precursors, they are economically disadvantageous since the rate of conversion and selectivity, and hence the yield, is relatively low; on the order of about 10%. U.S. Pat. No. 4,528,394 describes a method which improves upon this yield by using a benzaldehyde precursor, such that the yield is increased to about 50%. However, this system is disadvantageous because of the vigorous reaction conditions required and undesired side reactions which occur such as the self-condensation of the benzaldehyde. These may also undergo undesired oxidation to carboxylic acids under Baeyer-Villiger conditions. The present invention improves on these methods by preparing intermediates derived from certain ketones and conducting a Baeyer-Villiger oxidation thereon. The intermediates are prepared in a stepwise fashion and several advantages are thereby noted. These include a higher yield, perhaps in the 80-95% range, easier purification of the intermediates and less vigorous reaction conditions.